Pt complexes as phosphorescent emitters in the fabrication of organic light emitting diodes

ABSTRACT

A series of Pt(II) complexes having the following formula are disclosed:                  
         X 1  and X 2  independently are C or N, X 1  can also locate at another position of the hexagonal ring, when X 1  is N;   R 1  is H, C1–C8 alkyl, or C1–C4 perfluoroalkyl, R 2  is H, R 1  and R 2  together are C4–C8 alkylene, or R 1  and R 2  together are bridged carbocyclic C4–C12 alkylene, when X 2  is C;   R 1  is H, C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R 2  is omitted, when X 2  is N;   R 7  is H or methyl, and R 8  is omitted, when X 1  is N;   R 7  is H or methyl, R 8  is H or methyl, or R 7  and R 8  together are                  
 
when X 1  is C.

FIELD OF THE INVENTION

The present invention relates to an organic light emitting diode (OLED),particularly an OLED containing a Pt(II) complex as a phosphorescentemitter.

BACKGROUND OF THE INVENTION

Owing to their potential to harness the energies of both the singlet andtriplet excitons after charge recombination, transition metal basedphosphorescent materials have recently received considerable attentionin fabricating organic light-emitting diodes (OLEDs). The mainadvantages are due to the heavy atom induced singlet-to-tripletintersystem crossing as well as the large enhancement of radiative decayrate from the resulting triplet manifolds. In this regard, numerousattempts have been made to exploit third-row transition metal complexesas dopant emitters for OLED fabrication, among which quite a few Pt(II),Os(II) and Ir(III) complexes have been reported to exhibit highlyefficient device performances. Despite these developments, attempts tofurther expand the potential of the square planar Pt(II) complexes, inwhich the central metal ion possesses a higher atomic number than Os(II)and Ir(III) for efficient OLED applications, has encountered manyintrinsic obstacles. For example, the PtOEP (H₂OEP=octaethylporphyrin)type of emitter commonly has a ligand based phosphorescence withlifetimes as long as 30˜50 μs, so that saturation of emissive sites anda rapid drop in device efficiency at high drive current is observed.Also contributing to the poor device efficiency is the planar molecularconfiguration of many Pt(II) complexes, which leads to a stacking effectand hence the formation of aggregates or dimers that tend to formexcimers in the electronically excited state. To recognize the potentialof Pt(II) materials for applications in high efficiency OLEDs, therational design of Pt(II) complexes aimed at reduction of thephosphorescence radiative lifetime and the prevention of stackingbehaviour is critical.

SUMMARY OF THE INVENTION

In this invention, we disclose the design and synthesis of a new seriesof emissive Pt(II) complexes, in which the associated ligandchromophores possess a bulky, rigid architecture to effectively suppressthe aggregation effect. Moreover, drastic reduction of thephosphorescence radiative lifetime to several microseconds has also beenachieved due to the strong singlet-triplet state mixings. In one of thepreferred embodiments of the present invention, highly efficient Pt(II)OLEDs operating at 610˜630 nm have been successfully prepared for thefirst time, in which the dopant (Pt(II) complexes) concentrations, dueto the diminution of aggregation effect, can be substantially increasedto maximize the performance.

A primary objective of the present invention is to provide aphosphorescent Pt complex for use as a light emitter in the fabricationof an organic electroluminescent device.

Another objective of the present invention is to provide a series of Ptcomplexes for use as a light emitter in an electroluminescent devicecapable of emitting green, orange and red light.

The preferred embodiments of the phosphorescent Pt complex synthesizedaccording to the present invention include (but not limited to):

-   -   1. The phosphorescent Pt complex possessing the following        generalized structure:    -   wherein    -   X₁ and X₂ independently are C or N;    -   R₁ is H, C1–C8 alkyl, or C1–C4 perfluoroalkyl, R₂ is H, R₁ and        R₂ together are C4–C8 alkylene, or R₁ and R₂ together are        bridged carbocyclic C4–C12 alkylene, when X₂ is C;    -   R₁ is H, C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is        omitted, when X₂ is N;    -   R₇ is H or methyl, R₈ is H or methyl, or R₇ and R₈ together are        when X₁ is C;    -   R₇ is H or methyl, and R₈ is omitted, when X₁ is N; and X₁ may        locate at another position of the hexagonal ring, when X₁ is N.    -   2. The Pt(II) complex as defined in item 1, wherein X₁ is C, and        R₇ and R₈ together are    -   3. The Pt(II) complex as defined in Item 2, wherein X₂ is C.    -   4. The Pt(II) complex as defined in Item 3, wherein R₁ and R₂        together are bridged carbocyclic C4–C12 alkylene.    -   5. The Pt(II) complex as defined in Item 4, wherein R₁ and R₂        together are        wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.    -   6. The Pt(II) complex as defined in Item 5, wherein R₄, R₅, and        R₆ are methyl.    -   7. The Pt(II) complex as defined in Item 3, wherein R₁ is C1—C8        alkyl, or C1–C4 perfluoroalkyl, and R₂ is hydrogen.    -   8. The Pt(II) complex as defined in Item 2, wherein X₂ is N, R₁        is C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is omitted.    -   9. The Pt(II) complex as defined in Item 1, wherein X₁ is C, R₇        is H or methyl, and R₈ is H.    -   10. The Pt(II) complex as defined in Item 9, wherein X₂ is C.    -   11. The Pt(II) complex as defined in Item 10, wherein R₁ and R₂        together are bridged carbocyclic C4–C12 alkylene.    -   12. The Pt(II) complex as defined in Item 11, wherein R₁ and R₂        together are        wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.    -   13. The Pt(II) complex as defined in Item 12, wherein R₄, R₅ and        R₆ are methyl.    -   14. The Pt(II) complex as defined in Item 10, wherein R₁ is        C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is hydrogen.    -   15. The Pt(II) complex as defined in Item 10, wherein R₁ and R₂        together are C4–C8 alkylene.    -   16. The Pt(II) complex as defined in Item 15, wherein R₁ and R₂        together are tetramethylene.    -   17. The Pt(II) complex as defined in Item 9, wherein X₂ is N, R₁        is C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is omitted.    -   18. The Pt(II) complex as defined in Item 1, wherein X₁ is N, R₇        is H or methyl, and R₈ is omitted.    -   19. The Pt(II) complex as defined in Item 18, wherein X₂ is C.    -   20. The Pt(II) complex as defined in Item 19, wherein R₁ and R₂        together are bridged carbocyclic C4–C12 alkylene.    -   21. The Pt(II) complex as defined in Item 20, wherein R₁ and R₂        together are        are wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.    -   22. The Pt(II) complex as defined in Item 21, wherein R₄, R₅ and        R₆ are methyl.    -   23. The Pt(II) complex as defined in Item 19, wherein R₁ is        C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is hydrogen.    -   24. The Pt(II) complex as defined in Item 19, wherein R₁ and R₂        together are C4–C8 alkylene.    -   25. The Pt(II) complex as defined in Item 24, wherein R₁ and R₂        together are tetramethylene.    -   26. The Pt(II) complex as defined in Item 18, wherein X₂ is N,        R₁ is C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is omitted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a multi-layered OLED of the presentinvention.

FIG. 2 shows the X-ray structure of a Pt complex 1 synthesized inExample 1 according to the present invention.

FIG. 3 shows the UV-Vis absorption and cmission spectra of complex 1(2.68×10⁻⁵ M, -●-) and complex 2 (2.14×10⁻⁵ M, -Δ-) in CH₂Cl₂ at roomtemperature. Note that the normalized emission spectra were acquiredunder degassed condition. The dot lines denote the correspondingsolid-state emission obtained from a thin film sample at roomtemperature.

FIG. 4 a shows the phosphorescence spectra of complex 1 (-●-) andcomplex 2 (-Δ-) in the 77 K solid CH₂Cl₂ matrices.

FIG. 4 b shows the phosphorescence decay profiles of complex 1.

FIG. 4 c shows the phosphorescence decay profiles of complex 2.

FIG. 5 shows I–V characteristics of complex 1-based OLED devices as afunction of the dopant concentration.

FIG. 6 shows electroluminescence spectra of complex 1-based OLED devicesas a function of dopant concentrations.

FIG. 7 shows luminance efficiencies of complex 1-based OLED devices as afunction of current density.

FIG. 8 shows the X-ray structure of a Pt complex 3 synthesized inExample 3 according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following text, the synthesis and spectrum data of thephosphorescent Pt complexes according to the present invention aredescribed in detail, as well as the application of this type ofcomplexes as a phosphorescent material of an organic light-emittingdiode (OLED). The configuration of an OLED can be in a two layered,three layered, or multiple layered structures. FIG. 1 is a schematicdiagram of a multiple layered OLED device, wherein the actual thicknessof each layer is independent of the dimension depicted in the drawing.The structure of the multiple layered OLED device, in sequence,comprises a substrate (100), an anode (+), a hole injection modificationlayer (10), a hole transporting layer (20), an electron-blocking layer(not shown in the drawing), a light emitting layer (30), a hole-blockinglayer (40), an electron transporting layer (50), and a cathode (−). Saidelectron-blocking layer, hole injection modification layer (10), andhole-blocking layer (40), depending on the requirements of said device,may or may not be included in the structure thereof, wherein the layersbetween the positive electrode and the negative electrode constitute anelectroluminescent medium (400) of said device. Said light emittinglayer (30) is formed by doping a phosphorescence material as a dopantinto a host compound.

General procedures: All reactions were performed under nitrogen.Solvents were distilled from appropriate drying agents prior to use.Commercially available reagents were used without further purificationunless otherwise stated. All reactions were monitored by TLC with Merckpre-coated glass plates (0.20 mm with fluorescent indicator UV₂₅₄).Compounds were visualized with UV light irradiation at 254 nm and 365nm. Flash column chromatography was carried out using silica gel fromMerck (230–400 mesh). Mass spectra were obtained on a JEOL SX-102Ainstrument operating in electron impact (EI) or fast atom bombardment(FAB) mode. ¹H and ¹³C NMR spectra were recorded on a Bruker-400 orINOVA-500 instrument; chemical shifts are quoted with respect to theinternal standard tetramethylsilane for ¹H and ¹³C NMR data. Elementalanalysis was carried out with a Heraeus CHN-O Rapid Elementary Analyzer.

Spectroscopic and Dynamic Measurements: Steady-state absorption andemission spectra were recorded on a Hitachi (U-3310) spectrophotometerand an Edinburgh (FS920) fluorimeter, respectively. Bothwavelength-dependent excitation and emission response of the fluorimeterwere calibrated. A configuration of front-face excitation was used tomeasure the emission of the solid sample, in which the cell was made byassembling two edge-polished quartz plates with various Teflon spacers.A combination of appropriate filters was used to avoid interference fromthe scattering light. Lifetime studies were performed by an Edinburgh FL900 photon-counting system with a hydrogen-filled/or a nitrogen lamp asthe excitation source. Data were analyzed using a nonlinear leastsquares procedure in combination with an iterative convolution method.The emission decays were analyzed by the sum of exponential functions,which allows partial removal of the instrument time broadening andconsequently renders a temporal resolution of ˜200 ps.

To determine the photoluminescence quantum yield in solution, sampleswere degassed by three freeze-pump-thaw cycles under vigorous stirringconditions.4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM,λ_(em)=615 nm, Exciton, Inc.) in methanol was used as a reference,assuming a quantum yield of 0.43 with a 430 nm excitation. [J. M. Drake,M. L. Lesiecki, D. M. Camaioni, Chem. Phys. Lett. 1985, 113, 530.] Anintegrating sphere (Labsphere) was applied to measure the quantum yieldin the solid state, in which the solid sample film was prepared viaeither spin coating or vapor deposition methods and was excited by a 514nm (complex 1) or 457 nm (complex 2) Ar⁺ laser line. The resultingluminescence was led to an intensified charge-coupled detector forsubsequent quantum yield analyses. To obtain the PL quantum yield insolid state, the emission was collected via integrating sphere, and thequantum yield was calculated according to a reported method. [J. C. deMello, H. F. Wittmann, R. H. Friend, Adv. Mater. 1997, 9, 230.]

OLED Fabrication. Charge transporting materials such as NPB{4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl} and Alq₃[tris(8-hydroxyquinolinato)aluminium (III)], as well as the hostmaterial CBP (4,4′-N.N′-dicarbazolyl-1,1′-biphenyl) were synthesizedaccording to literature procedures, [A. Y. Sonsale, S. Gopinathan, C.Gopinathan, Indian J. Chem. 1976, 14, 408; B. E. Koene, D. E. Loy, M. E.Thompson, Chem. Mater. 1998, 10, 2235.] and were sublimed twice througha temperature-gradient sublimation system before use. BCP(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) was obtained fromAldrich. Patterned ITO-coated glass substrates (sheet resistance≦30 Ω/□)with an effective individual device area of 3.14 mm² were cleaned bysonication in a detergent solution, water and ethanol, respectively andthen dried by a flow of nitrogen. The substrates were further treatedwith oxygen plasma for 3 min before loading into the vacuum chamber.Various organic layers were deposited sequentially at a rate of 0.1˜0.3nm/s under a pressure of 2×10⁻⁵ Torr in an Ulvac Cryogenic depositionsystem. Phosphorescent dopants were co-evaporated with CBP via twoindependent sources. A thin layer of LiF (1 nm) and a thick layer of Al(150 nm) were followed as the cathode. The current-voltage-luminance ofthe devices was measured in ambient conditions with a Keithley 2400Source meter and a Newport 1835C Optical meter equipped with 818STsilicon photodiode. The EL spectrum was obtained using a HITACHI F4500spectrofluorimeter. The active area of the device was 3.14 mm² and thatof the silicon photodiode was 100 mm². The device was placed close tothe photodiode such that all the forward light goes to the photodiode.The external quantum efficiency was calculated according to the methoddescribed before. [S. R. Forrest, D. D. C. Bradley, M. E. Thompson, Adv.Mater. 2003, 15, 1043.] The luminous flux(lm) is defined byP_(v)=K_(m)∫_(λ)P_(e,λ)V(λ)dλ; where K_(m) is the maximum luminousefficiency (683 lm/W), P_(e,λ) is the spectral concentration of radiantflux, V(λ) is the relative photopic luminous efficiency function [G.Wyszecki, W. S. Stiles, “Color Science: Concepts and Methods,Quantitative Data and Formulae” John Wiley & Sons, New York, 1982. p.259.]; the luminance (cd/m²) is defined by luminous flux/πa, where a isthe device area; the luminous efficiency (cd/A) is defined by luminousflux/πI, where I is the current; power efficiency is defined as luminousflux/IV, where V is the applied voltage.

Synthesis. The multi-step reactions giving the first two Pt(II) emittingcomplexes are shown in Scheme 1.

These Pt(II) complexes exhibit enhanced emission quantum yields, shortphosphorescence radiative lifetimes in the range of several microsecondsand, more importantly, a much lower tendency in forming aggregation thanpreviously reported Pt(II) porphyrinato or β-diketonato complexes forelectroluminescent applications. [R. C. Kwong, S. Sibley, T. Dubovoy, M.Baldo, S. R. Forrest, M. E. Thompson, Chem. Mater. 1999, 11, 3709; C.-M.Che, Y.-J. Hou, M. C. W. Chan, J. Guo, Y. Liu, Y. Wang, J. Mater Chem.2003, 13, 1362.] Remarkable improvement of the device performances hasbeen achieved at higher dopant concentrations or even in a pure emissionlayer, constituting for the first time a highly efficient Pt(II)-basedOLED in the red.

Characterization. Both Pt complexes, Pt(iqdz)₂ (1) and Pt(pydz)₂ (2),were highly soluble in organic solvents and have been characterizedusing various spectroscopic methods (see experimental section). Theirspectroscopic data are in good agreement with the expected square planarPt(II) complexes coordinated with two indazole chelates. Complex 1 wasfurther examined by single crystal X-ray diffraction analysis toestablish its molecular structure. FIG. 2 depicts the X-ray molecularstructure of the complex 1.

Photophysical Measurements. As shown in FIG. 3, although the energy gapsare quite different, similar spectral features are observed forcomplexes 1 and 2, consisting of a weak, broad band located in the longwavelength region, accompanied by a vibronic progression feature in theshort wavelength region that is commonly assigned to the singlet π—π*intra-ligand transitions (¹IL). The low frequency absorption bands havea relatively small extinction coefficient (1: 5240 M⁻¹cm⁻¹ at 502 nm; 2:4690 M⁻¹cm⁻¹ at 457 nm) are tentatively assigned to the transitionincorporating a state mixing amongst singlet and triplet metal-ligandcharge transfer (¹MLCT and ³MLCT) and, to a certain extent, theintra-ligand triplet state (³IL). Support for this viewpoint is firstprovided by a distinct shoulder at 530 nm (ε˜4080 M⁻¹ cm⁻¹) resolved inthe absorption spectrum of complex 1, which can be tentatively assignedto the lowest lying ³MLCT band (vide infra). The close energetics andabsorptivity between the ¹MLCT and ³MLCT bands suggest that the ³MLCTtransition, induced by spin-orbit coupling and the proximal energylevels with respect to ¹MLCT, is greatly enhanced and becomes partiallyallowed. This novel spectral feature is in accord with data for otherrecently published Pt(II) complexes bearing bis(phenoxy)diimineauxiliaries capable of tetradentate bonding. [Y.-Y. Lin, S.-C. Chan, M.C. W. Chan, Y.-J. Hou, N. Zhu, C.-M. Che, Y. Liu, Y. Wang, Chem. Eur. J.2003, 9, 1263.] Although the authors have not explicitly examined anddiscussed the details of the singlet-triplet mixing, its presence isunambiguously confirmed by the obvious overlap between this UV-Visabsorption band and the leading edge of the corresponding emissionprofile.

As for complex 2, the ¹MLCT and ³MLCT states are so close that anasymmetric band was observed rather than the well-resolved dualabsorption profile. Complex 1 exhibits an intensive emission maximizedat 635 nm (Φ=0.81; τ=5.34 μs) in degassed CH₂Cl₂. The oxygen quenchingrate of 1.78×10⁹ M⁻¹ s⁻¹ for the emission in CH₂Cl₂, in combination withits spectral mirror image with respect to the lowest absorption profile,leads us to conclude that the emission mainly originates from a tripletmanifold. Similarly, complex 2 also exhibits strong phosphorescence witha peak wavelength at 553 nm (Φ=0.64; τ=3.63 μs), which is comparable tothose observed in the cyclometalated dipyridylbenzene Pt complexes. [J.A. G. Williams, A. Beeby, E. S. Davies, J. A. Weinstein, C. Wilson,Inorg. Chem. 2003, 42, 8609.] The observed radiative lifetimes for 1 and2, respectively, in CH₂Cl₂ are relatively long for a pure ³MLCT emissionfrom complexes incorporating a central heavy atom like Pt(II) and a¹MLCT state in proximity. Accordingly, we tentatively propose that thereexists, in part, a further state mixing with the ³IL manifold. Firmsupport for this viewpoint is given by the unusually broad,structureless emission feature, with a full-width-at-half-maximum (fwhm)of 108 nm and 109 nm for 1 and 2, respectively in CH₂Cl₂ at RT, whileupon cooling to 77 K the emission reveals distinctive vibronic-likefeatures with peak wavelengths at 580, 614 (520), 656 (552) and 698(596) nm for complex 1 and 2 (see FIG. 4 and Table 1), respectively.Although not well resolved, similar structural features were alsoobserved for both complexes in the solid state at RT (see FIG. 3). Thespectral progression of >1000 cm⁻¹ for each successive peak cannot berationalized by the much smaller d level splitting in a square planarcoordination, but is akin to that corresponding to the vibrational modes(1270˜1300 cm⁻¹) of aromatic terpyridyl ligands. [Q.-Z. Yang, L.-Z. Wu,Z.-X. Wu, L.-P. Zhang, C.-H. Tung, Inorg. Chem. 2002, 41, 5653; V. W.-W.Yam, R. P.-L. Tang, K. M.-C. Wong, K.-K. Cheung, Organometallics 2001,20, 4476.] Alternatively, it may be plausible that the broad fwhm,together with the vibronic structure features in a 77 K CH₂Cl₂ solutionas well as in the solid state at RT, arises from a state mixing between³MLCT and ³IL. [J. DePriest, G. Y. Zheng, N. Goswami, D. M. Eichhorn, C.Woods, D. P. Rillema, Inorg. Chem. 2000, 39, 1955; G. Y. Zheng, D. P.Rillema, J. DePriest, C. Woods, Inorg. Chem. 1998, 37, 3588; G. Y.Zheng, D. P. Rillema, Inorg. Chem. 1998, 37, 1392.] In a central planarconfiguration like 1 and 2, the strong mixing of these two transitionsessentially requires covalent interaction of the relevant d orbitals andthe ligand π system, in which MLCT [d_(xz,yz)→π*] transitions should bethe most likely candidates. [W. B. Connick, V. M. Miskowski, V. H.Houlding, H. B. Gray, Inorg. Chem. 2000, 39, 2585.]

Due to the planar geometry of the central Pt(II) atom possessing a dsp²configuration, one has to consider the possible stacking effects forboth 1 and 2. We thus carried out a concentration-dependentabsorption/emission study in an attempt to resolve this issue. Uponvarying the sample concentrations from 3.45×10⁻⁵ M to 1.07×10³ M, bothabsorption and emission spectra remain unchanged for 1 and 2, indicatingthat the stacking effect, if any, is too small to affect any associatedphotophysical behaviour. It is thus reasonable to conclude that theintroduction of a bulky camphor derived group on the indazole fragmentdrastically increases the steric hindrance and hence suppressesaggregation. Supplementary support for this viewpoint is provided by thesolid-state emission spectra in that both complexes (1: λ_(em)=638 nm;2: λ_(em)=551 nm, see FIG. 3) manifest negligible spectral shifts fromthe corresponding emission maximum in solution, providing unambiguousevidence for the negligible stacking interaction in both complexes. Itis noted that the solid film PL is narrower than the solution PL. Inaddition, the PL spectra in the solid film show slight blue shift for 2and red shift for 1 in comparison with their corresponding emission insolution. If the solid state of a complex lacks strong intermolecularinteraction such as hydrogen bonding, π stacking, etc., a slightly blueshift and narrowness for the solid film PL relative to that of thesolution PL may be expected, and can be attributed to a “medium effect”For complex 2 in solution, the stronger interaction from the solvent(e.g. CH₂C₂) makes the emission broader, whereas due to the lack of πstacking, complex 2 is more or less frozen and inhibited from having acloser interaction with each other. In comparison, the additional fusedbenzene of isoquinoline in complex 1 introduces a weak but perhapsnon-negligible π interaction in the solid film, resulting in a slightlyred shifted emission. Nevertheless, from solution to solid, the shift ofpeak wavelength is rather small for 1, indicating that theintermolecular interaction cannot be large. This viewpoint can besupported from X-ray single crystal analysis, in which a rather long Pt. . . Pt distance has been resolved for 1 (vide supra).

Table 1 lists detailed spectroscopy and dynamics data for 1 and 2 insolution and in a single crystal. Despite the much lower energy gap withrespect to that of the pyridyl counterpart 2, significant enhancement ofthe luminescent quantum yield (Φ˜0.81) is observed in 1, accompanied bya longer lifetime (5.34 μs) in degassed CH₂Cl₂. These results seem tocontradict the energy-gap law, in which the theory pertaining toradiationless decay concludes that the radiationless deactivation shouldincrease upon decreasing the energy gap of the transition. [S. R.Johnson, T. D. Westmoreland, J. V. Caspar, K. R. Barqawi, T. J. Meyer,Inorg. Chem. 1988, 27, 3195; C. E. Whittle, J. A. Weinstein, M. W.George, K. S. Schanze, Inorg. Chem. 2001, 40, 4053.] We thus tentativelypropose that the remarkable but unusual luminescence behaviour incomplex 1 is due to the highly conjugated π systems in indazole coupledwith the nearby isoquinoline fragments. One possible strategy tosuppress the active vibrational modes, such as ring stretching andbending of the acceptor ligand that commonly dominates the deactivationof the MLCT excited states, is to use a ligand with a rigid σ-framework.In addition, upon excitation, the electron occupation of the lowest π*acceptor orbital results in increases of the C—C and C—N bond distances,inducing radiationless transition due to the loose bonding effect. [J.V. Caspar, T. D. Westmoreland, G, H. Allen, P. G. Bradley, T. J. Meyer,W. H. Woodruff, J. Am. Chem. Soc. 1984, 106, 3492; E. M. Kober, T. J.Meyer, Inorg. Chem. 1985, 24, 106.] Since the Franck-Condon factors fornonradiative transitions is qualitatively proportional to the square ofthe bonding displacement, enlarging the π conjugation should reduce thedistortion of the ligand framework due to the smaller changes in averagedistance between ground and excited states. [A. El-Ghayoury, A.Harriman, A. Khatyr, R. Ziessel, Angew. Chem., Int. Ed. 2000, 39, 185;P. A. Anderson, F. R. Keene, T. J. Meyer, J. A. Moss, G. F. Strouse, J.A. Treadway, J. Chem. Soc. Dalton Trans. 2002, 3820; Y.-Q. Fang, N. J.Taylor, G. S. Hanan, F. Loiseau, R. Passalacqua, S. Campagna, H.Nierengarten, A. Van Dorsselaer, J. Am. Chem. Soc. 2002, 124, 7912.]Accordingly, the radiationless decay rate in 1 is expected to berelatively small to compete with the red phosphorescence, giving rise toan exceptionally high emission quantum yield. For comparison,Nazeeruddin and co-workers have recently reported near unity quantumyields for the blue, green and yellow emission by meticulous selectionof the ligand on the Ir(III) system possessing strong ligand fieldstrength, which increased the energy gaps between triplet emittingstates and the nearby deactivating MC level. [M. K. Nazeeruddin, R.Humphry-Baker, D. Berner, S. Rivier, L. Zuppiroli, M. Graetzel, J. Am.Chem. Soc. 2003, 125, 8790.] Other factors leading to the high emissionyield for 1 or 2 are also possible. These include (a) Pt(II) metal ionintrinsically possessing a relatively larger d-orbital splitting, (b)isoquniolinyl indazole (or pyridyl indazole) with a fairly strong ligandfield inducing a larger gap between the MC states and the LUMO of theligands and (c) close lying π—π* and MLCT states together with the heavyatom effect enhancing the spin-orbital coupling. A comprehensiveunderstanding of the relaxation mechanisms might have to rely on futuretheoretical approaches, focus of which is currently in progress.

OLED Fabrication. Due to its high phosphorescence quantum efficiency inthe red, multilayer devices of the configuration ITO/NPB(40 nm)/CBP:1(30nm)/BCP(10 nm)/Alq₃(30 nm)/LiF(1 nm)/Al(150 nm) were prepared, withdoping concentrations of 1 varying from 6%, 12%, 20%, 50% to a neatfilm. Very bright red emission was observed for all the concentrationsprepared, including the one with a pure layer of the platinum complex.The I–V curves, plotted in FIG. 5, show a trend of increasing currentdensity with increasing concentrations of 1. The results may imply thatthe phosphorescent sites serve as charge trapping sites. The EL spectraoriginated solely from the complex in all cases, but with a small redshift of the EL spectra λ_(max) upon increasing the dopantconcentrations, being 610 and 630 nm for the 6% dopant concentration andthe neat film, respectively (FIG. 6). The fwhm of the EL spectrum alsoincreased slightly (from 76 nm to 92 nm) over the same range of dopingconcentrations. Comparing FIGS. 3 and 6, the EL from the device isslightly blue shifted from PL of the solid film. One possible origin forthe shift is from the microcavity effect, which is caused by theinterference between the forwarding light and the light reflected fromthe metal electrode. [A. Dodabalapur, L. J. Rothberg, T. M. Miller, E.W. Kwock, Appl. Phys. Lett. 1994, 64, 2486; S. K. So, W. K. Choi, L. M.Leung, K. Neyts, Appl. Phys. Lett. 1999, 74, 1939; Y. Fukuda, T.Watanabe, T. Wakimoto, S. Miyaguchi, M. Tsuchida, Synth. Met. 2000,111–112, 1.]

All devices showed a turn-on voltage of as low as 4.0 V. Although thedevices exhibited a similar dropping trend with increasing current (FIG.7), as is the case for most phosphorescence-based devices, theperformance characteristics are nevertheless very encouraging. For thedevice doped with 12% of 1 driven at a current of 100 mA, a brightnessof 10677 cd/m² was achieved with an external quantum efficiency of ˜7%,a luminance efficiency of ˜11 cd/A and a power efficiency of 3.3 lm/W.The results also exhibited a decreasing trend with increasingconcentration of the platinum dopant. However, it is noteworthy thateven using a pure film of 1 as the emission layer, a brightness of 2653cd/m² and an external quantum efficiency of 2.46%, luminance efficiencyof 2.65 cd/A and power efficiency of 0.93 lm/W can be achieved. Therelatively high efficiency of the device even in the neat complex istentatively attributed to the unusually short radiative lifetime thatavoids the triplet—triplet annihilation. Table 2 summarizes theperformance data for various concentrations studied. The achievement ofhigh luminescence efficiency can be attributed to a much shorterphosphorescence radiative lifetime in combination with a rationallydesigned structure that greatly suppresses the aggregation effect. Itshould be noted that rigid steric blockers such as a pinenefunctionality incorporated into an octahedral phenyl pyridine Ir(III)complex have been reported to effectively reduce self-quenching of thephosphorescent dopant. [H. Z. Xie, M. W. Liu, 0. Y. Wang, X. H. Zhang,C. S. Lee, L. S. Hung, S. T. Lee, P. F. Teng, H. L. Kwong, H. Zheng, C.M. Che, Adv. Mater. 2001, 13, 1245.] While improving the deviceluminescence efficiency, this prior innovation based on Ir(ppy)₃ and itspinene derivatized complexes exhibits green emission rather than themuch needed, saturated red emission.

Moreover, the phosphorescent OLEDs with configuration ITO/NPB(40nm)/CBP:Pt(II) (30 nm)/BCP(10 nm)/Alq₃(30 nm)/LiF(1 nm)/Al(150 nm) arealso prepared using the green emitting Pt(II) complexes 2 and 5,respectively. Their device performance characteristics are summarized inTables 3 and 4. As indicated from the data, bright green emission wasobserved for all the concentrations prepared, showing max. brightness of˜73000 cd/m² with the highest external quantum efficiency(η_(ext))˜6.6%, which are comparable to the best performingcyclometalated complex Ir(ppy)₃.

TABLE 1 Photophysical properties of selected Pt(II) complexes indegassed CH₂Cl₂ at RT (298 K). λ_(abs) ^(max) (ε, M⁻¹cm⁻¹) PL λ_(max)[nm] Φ τ [μs] Pt(iqdz)₂ (1) 502, (5240) 635 0.81 5.34 (610, 638,688)^(a) 0.20^(a) 1.10^(a,c) (580, 614, 656, 698)^(b) 3.0^(b) Pt(pydz)₂(2) 457, (4690) 553 0.64 3.63 (528, 550, 581)^(a) 0.15^(a) 3.38^(a,c)(520, 552, 596)^(b) 2.78^(b) Pt(iq3dz)₂ (3) 450, (1996) 587 0.24 1.65Pt(bqpz)₂ (8) 466, (5286) 584, 631 0.74 13.0 ^(a)The solid-stateemission obtained from a thin film sample at RT. ^(b)The phosphorescentemission recorded in frozen CH₂Cl₂ matrices at 77 K. ^(c)Lifetime is anaverage value from a two-component fit.

TABLE 2 Performance characteristics for the OLED devices based oncomplex 1. Power Doping Brightness Lum. Eff. Eff. V_(drive) V_(turn-on)FWHM CIE conc. (cd/m²) η_(ext) (%) (Cd/A) (lm/W) (V) (V) (nm) (x, y)  6% 3451^(a) 10.6  17.43 6.37 8.61 4 610/76 0.61, 0.38 10846^(b) 6.6  10.923.24 10.61 33193(15)^(c) 14.9(5.5) 24.57(5.5) 14.86(5.0) 12%  3210^(a)10.51  16.21 6.20 8.24 3.8 612/78 0.62, 0.37 10677^(b) 6.98 10.76 3.2810.32 33394(15)^(c) 12.8(6.0) 19.79(6.0) 12.43(4.5) 20%  2378^(a) 8.2811.97 4.95 7.6 3.5 616/80 0.63, 0.37  8592^(b) 5.95  8.59 2.84 9.533454(15)^(c) 10.2(5.0)  14.7(5.0)  9.28(4.5) 50%  1397^(a) 6.23  7.042.87 7.71 3.6 626/88 0.64, 0.36  5101^(b) 4.55  5.14 1.68 9.6319430(15)^(c) 8.00(4.5)  9.03(4.5)  6.31(4.5) 100%   694^(a) 3.24 3.51.52 7.23 3.8 630/92 0.64, 0.35  2653^(b) 2.46  2.65 0.93 9.0110733(15)^(c) 3.91(4.5)  4.22(4.5)  2.95(4.5) ^(a)values collected under20 mA/cm² ^(b)values collected under 100 mA/cm² ^(c)max values of thedevices; values in the parentheses are the voltages at which they wereobtained.

TABLE 3 Performance characteristics for the OLED devices based oncomplex 2. Power Doping Brightness Lum. Eff. Eff. V_(drive) V_(turn-on)FWHM CIE conc. (cd/m²) η_(ext) (%) (Cd/A) (lm/W) (V) (V) (nm) (x, y)  6% 4657^(a) 6.59 23.34 9.89 7.42 3.6 542/78 0.35, 0.56 17453^(b) 4.9517.54 5.88 9.39 59807(15)^(c) 7.10(6.5) 25.13(6.5) 13.58(5.0) 50% 3890^(a) 5.53 19.60 12.56  4.92 2.7 556/82 0.41, 0.55 14481^(b) 4.1214.60 7.22 6.37 53663(15)^(c) 6.54(3.5) 23.17(3.5) 20.83(3.5) 100%  4110^(a) 6.14 20.65 14.29  4.55 2.6 568/82 0.45, 0.52 16152^(b) 4.8416.25 8.36 6.12 54628(15)^(c) 6.52(3.5) 21.9(3.5) 19.68(3.5) ^(a)valuescollected under 20 mA/cm² ^(b)values collected under 100 mA/cm² ^(c)maxvalues of the devices; values in the parentheses are the voltages atwhich they were obtained.

TABLE 4 Performance characteristics for the OLED devices based oncomplex 5. Power Doping Brightness Lum. Eff. Eff. V_(drive) V_(turn-on)FWHM CIE conc. (cd/m²) η_(ext) (%) (Cd/A) (lm/W) (V) (V) (nm) (x, y)  6% 2357^(a) 4.19 11.82 5.47 6.80 4.0 502/60 0.19, 0.50  9937^(b) 3.54 9.99 3.75 8.39 44656(15)^(c) 4.23(6.5) 11.95(6.5) 6.37(5.5) 50% 1484^(a) 2.19  7.40 4.33 5.39 3.1 520/74 0.28, 0.58  7141^(b) 2.12 7.16 3.34 6.74 38960(15)^(c) 2.20(5.5) 7.44(5.5) 4.82(4.0) 100%  4602^(a) 6.54 23.02 16.10  4.49 2.8 542/96 0.37, 0.57 19402^(b) 5.5719.60 10.81  5.72 73342(15)^(c) 6.64(4.0) 23.38(4.0) 18.37(4.0)^(a)values collected under 20 mA/cm² ^(b)values collected under 100mA/cm² ^(c)max values of the devices; values in the parentheses are thevoltages at which they were obtained.

EXPERIMENTS Example 1 Synthesis of4,8,8-Trimethyl-3-isoquinoline-1-yl-4,5,6,7-tetrahydro-2H-4,7-methano-indazole,(iqdz)H

To a stirred mixture of NaH (0.26 g, 10.8 mmol) and THF (10 mL) at 0° C.was added a solution of (1R)-(+)-camphor (1.64 g, 10.8 mmol) in THF fora period of 10 min. The temperature of the reaction mixture was slowlyincreased to RT and stirring was continued for about 30 min. Then thesolution was heated to 60° C., and ethyl 1-isoquinolinecarboxylate (1.7g, 8.5 mmol) in THF was added slowly and refluxed for about 3 h. Afterthis period, the reaction mixture was cooled to 0° C. and quenched withdilute HCl until pH=8–9. Then it was extracted with ethyl acetate (2×100mL), and the extracts were washed with brine, and water, dried overanhydrous MgSO₄ and concentrated in vacuo to give a yellow oil (2.2 g).Without further purification, to a refluxing solution of the above oil(2.2 g) in EtOH (30 mL) was added dropwise hydrazine monohydrate (4.2mL, 86.0 mmol) in EtOH. After the mixture was refluxed for 12 h, thesolvent was removed under vacuum. The residue obtained was dissolved inethyl acetate and washed with water, dried over anhydrous MgSO₄ andconcentrated again. The residue obtained was passed through a silica gelcolumn using mixtures of hexane and ethyl acetate as eluents to give(iqdz)H as colorless crystals (1.4 g, 55%).

Spectral data: MS (EI), m/z 303, M⁺. ¹H NMR (500 MHz, CDCl₃, 294 K): δ8.51 (d, J=5.8 Hz, 1H), 8.42 (d, J=8.3 Hz, 1H), 7.81 (d, J=8.4 Hz, 1H),7.67 (ddd, J=8.3, 6.8, 1.3 Hz, 1H), 7.61 (ddd, J=8.4, 6.8, 1.5 Hz, 1H),7.56 (d, J=5.8 Hz, 1H), 3.00 (d, J=4.0 Hz, 1H), 2.19 (m, 1H), 1.92 (m,1H), 1.44 (m, 2H), 1.35 (s, 3H), 0.98 (s, 3H), 0.79 (s, 3H). ¹³C NMR(125 MHz, CDCl₃, 294 K): δ 167.1, 149.9, 141.8, 136.8, 132.5, 130.2,127.3, 127.0, 126.3, 126.2, 125.5, 120.1, 61.0, 50.4, 50.0, 33.6, 27.5,20.5, 19.2, 10.6. Anal. Calcd. for C₂₀H₂₁N₃: C, 79.17; H, 6.98; N,13.85. Found: C, 79.49; H, 6.98; N, 13.92.

Synthesis of Pt(iqdz)₂ (1)

A solution of potassium tetrachloroplatinate (K₂PtCl₄) (0.1 g, 0.24mmol), (iqdz)H (0.16 g, 0.53 mmol) in a mixture of ethanol (15 mL) andwater (5 mL) was heated at 80° C. for about 16 h. After this period, thereaction mixture was cooled and the precipitated solid was filtered off,washed with ether and dried under vacuum to give Pt(iqdz)₂ as a redsolid (1, 0.15 g, 78%). Crystals of 1 suitable for X-ray analysis wereobtained by recrystallization from a mixture of dichloromethane andhexane at room temperature.

Spectra data of 1: MS (FAB), m/z 800, M⁺. ¹H NMR (400 MHz, CD₂Cl₂, 294K): δ 10.93 (d, J=6.4 Hz, 2H), 8.86 (d, J=8.2 Hz, 2H), 7.94 (d, J=7.8Hz, 2H), 7.86 (dd, J=8.2, 7.0 Hz, 2H), 7.76 (dd, J=7.8, 7.0 Hz, 2H),7.63 (d, J=6.4 Hz, 2H), 3.44 (d, J=2.8 Hz, 2H), 2.34 (m, 2H), 2.02 (m,2H), 1.58˜1.47 (m, 4H), 1.52 (s, 6H), 1.09 (s, 6H), 0.85 (s, 6H). ¹³CNMR (100 MHz, CD₂Cl₂, 294 K): δ 164.8, 156.5, 144.2, 142.1, 136.9,132.1, 128.2, 127.8, 127.6, 126.9, 123.8, 118.4, 60.8, 52.8, 50.7, 34.1,27.9, 20.5, 19.4; 10.9 Anal. Calcd for C₄₀H₄₀N₆Pt; C, 60.43; H, 5.32; N,10.31. Found: C, 60.52; H, 5.29; N, 10.58.

Example 2 Synthesis of4,8,8-Trimethyl-3-pyridin-2-yl-4,5,6,7-tetrahydro-2H-4,7-methano-indazole,(pydz)H

Using the same conditions as for (iqdz)H, starting from ethyl picolinateand (1R)-(+)-camphor, the title compound was obtained as white crystals(yield 34%).

Spectral data: MS (EI), m/z 253, M⁺. ¹H NMR (500 MHz, CDCl₃, 294 K): δ8.58 (d, J=5.5 Hz, 1H), 7.69 (ddd, J=7.8, 7.5, 1.8), 7.53 (d, J=7.8 Hz,1H), 7.15 (ddd, J=7.5, 5.5, 1.3 Hz, 1H), 3.03 (d, J=4.5 Hz, 1H), 2.14(m, 1H), 1.87 (m, 1H), 1.35 (m, 1H), 1.24 (m, 1H) 1.31 (s, 3H), 0.98 (s,3H), 0.71 (s, 3H). ¹³C NMR (125 MHz, CDCl₃, 294 K): δ 167.6, 149.2,149.1, 136.9, 133.5, 124.6, 122.1, 120.5, 61.2, 50.3, 48.2, 33.5, 27.2,20.5, 19.3, 10.5. Anal. Calcd. for C₁₆H₁₉N₃: C, 75.85; H, 7.56; N,16.59. Found: C, 76.07; H, 7.48; N, 16.60.

Synthesis of Pt(pydz)₂ (2)

Using the same conditions as for Pt(iqdz)₂ (1), starting from K₂PtCl₄and the ligand (pydz)H, the title compound 2 was obtained as a yellowpowder (yield 50%).

Spectral data of 2: MS (FAB), m/z 700, M⁺. ¹H NMR (500 MHz, CDCl₃, 294K): δ 10.72 (brs, 2H), 7.82 (dd, J=7.5, 7.5 Hz, 2H), 7.52 (d, J=8.0 Hz,2H), 7.22 (brs, 2H), 3.03 (d, J=4.0 Hz, 2H), 2.13 (m, 2H), 1.86 (m, 2H),1.35–1.44 (m, 2H), 1.19 (m, 2H), 1.42 (s, 6H), 97 (s, 6H), 0.78 (s, 6H).¹³C NMR (125 MHz, CDCl₃, 294 K): δ 164.5, 155.0, 152.8, 142.5, 138.7,126.1, 120.9, 118.2, 61.8, 50.9, 47.9, 33.8, 27.8, 20.8, 19.7, 11.2.Anal. Calcd. for C₃₂H₃₆N₆Pt: C, 54.93; H, 5.19; N, 12.01. Found: C,54.85; H, 5.22; N, 11.91.

Selected crystal data of 1: C₄₀H₄₀N₆Pt, M=799.87, monoclinic, spacegroup P 2₁/n, a=6.8567(3), b=18.4625(9), c=15.5327(8) Å, β=95.824(1)°,V=1956.6(16) Å³, Z=2, ρ_(calcd)=1.358 mgm⁻³, F(000)=800, crystalsize=0.40×0.10×0.03 mm, λ(Mo—K_(α))=0.7107 Å, T=295(2) K, μ=3.620 mm⁻¹,18030 reflections collected (R_(int)=0.0467), final R₁[I>2σ(I)]=0.0573and wR₂(all data)=0.1552.

Example 3 Synthesis of Pt(iq3dz)₂ (3)

A solution of potassium tetrachloroplatinate (K₂PtCl₄) (0.1 g, 0.24mmol),3-isoquinoline-3-yl-7,8,8-trimethyl-4,5,6,7-tetrahydro-2H-4,7-methano-indazole,(iq3dzH, 0.16 g, 0.53 mmol) in a mixture of ethanol (15 mL) and water (5mL) was heated at 80° C. for about 16 h. After cooling the mixture toroom temperature, the precipitate was collected, washed with diethylether and dried under vacuum to give yellow solid 52% (0.1 g, 0.13mmol). Crystals of Pt(iq3dz)₂ suitable for X-ray analysis were obtainedby recrystallization from a mixture of dichloromethane and hexane atroom temperature.

Spectra data of (3): MS (FAB), observed m/z (actual) [assignment]: 800(800) [M⁺]. ¹H NMR (400 MHz, CDCl₃, 294 K): δ 11.78 (s, 2H), 8.16 (d,J=8.0 Hz, 2H), 7.81–7.73 (m, 6H), 7.55 (ddd, J=6.8, 6.6,1.3 Hz, 2H),3.17 (d, J=4.0 Hz, 2H), 2.20 (m, 2H), 1.92 (m, 2H), 1.50 (m, 6H),1.46(2H, m),1.29 (m, 2H), 1.03 (s, 6H), 0.85 (s, 6H). ¹³C NMR (125 MHz,CDCl_(3,) 294 K): δ 168.9 (2C), 157.3 (2C), 148.3 (2C), 142.2 (2C),136.7 (2C), 132.9 (2C), 129.7 (2C),126.9 (2C),126.4 (2C), 126.1 (2C),124.2 (2C), 113.4 (2C), 61.7 (2C), 50.7 (2C), 48.0 (2C), 34.0 (2C), 27.9(2C), 20.9 (2C), 19.8 (2C), 11.2 (2C). Anal. Calcd. for C₄₀H₄₀N₆Pt: C,60.43; H, 5.32; N, 10.31. Found: C, 60.52; H, 5.29; N, 10.58.

Selected crystal data of (3): C₄₀H₄₀N₆Pt, M=799.87, orthorhombic, spacegroup P 2₁/n, a=7.0868(3), b=16.9811(8), c=30.3358(15) Å, β=90.0 (1)°,V=3650.7(3) Å³, Z=4, ρ_(calcd)=1.455 mgm ⁻³, F(000)=1600, crystalsize=0.25×0.12×0.05 mm, λ(Mo—K_(a))=0.7107 Å, T=295(2) K, μ=3.879 mm⁻¹,43030 reflections collected (R_(int)=0.0452), final R₁[I>2σ(I)]=0.0553and wR₂(all data)=0.1280. FIG. 8 depicts the X-ray molecular structureof the complex 3.

Example 4 Synthesis of Pt(mppz)₂ (4)

To the suspension of NaH (15 mg, 0.63 mmol) in 15 mL of THF was added3-methyl-5-(2-pyridyl) pyrazole (mppzH, 83 mg, 0.52 mmol) slowly at roomtemperature. After being stirred for 1.5 hour, the solution was filteredand transferred into a reaction flask loaded with 100 mg of Pt(DMSO)₂Cl₂(0.24 mmol) and 10 mL of THF. The solution was refluxed for 12 hours.After then, the solvent was evaporated under vacuum and 60 mL of CH₂Cl₂was added to extract the product. The extract was then washed withwater, dried with anhydrous Na₂SO₄, giving a yellowish green materialafter removal of all solvent. Further purification was conducted usingvacuum sublimation (160° C., 220 mtorr), affording 80 mg of Pt(mppz)₂ asbright green solid (0.16 mmol, 65%).

Spectra data of (4): MS (EI, 70 eV), observed m/z (actual) [assignment]:511 (511) [M⁺]. ¹H NMR (500 MHz, CDCl₃, 294 K): δ 10.64 (d, ³J_(HH)=6.0Hz, 2H), 7.86 (t, ³J_(HH)=8.0 Hz, ³J_(HH)=8.0 Hz, 2H), 7.55 (d,³J_(HH)=8.0 Hz, 2H), 7.28 (t, ³J_(HH)=6.0 Hz, ³J_(HH)=6.0 Hz, 2H), 6.43(s, 2H), 2.45 (s, 6H). ¹³C NMR (125 MHz, CDCl₃, 294K): δ 154.7 (C_(py)),152.5 (C_(pz)), 150.3 (CH_(py)), 147.6 (C_(pz)), 139.1 (CH_(py)), 121.6(CH_(py)), 118.2 (CH_(py)), 103.0 (CH_(pz)), 14.0 (CH₃). Anal. Calcd.for C₁₈H₁₆N₆Pt: C, 42.27; N, 16.43; H, 3.15. Found: C, 42.08; N, 16.38;H, 3.49.

Example 5 Synthesis of Pt(bppz)₂ (5)

To the suspension of NaH (14 mg, 0.58 mmol) in 15 mL of THF was added3-tert-butyl-5-(2-pyridyl) pyrazole (bppzH, 110 mg, 0.54 mmol) slowly atroom temperature. After being stirred for 1.5 hour, the solution wasfiltered and transferred into a flask that loaded with 100 mg ofPt(DMSO)₂Cl₂ (0.24 mmol) and 10 mL of THF. The solution was refluxed for12 hours. After then, the solvent was evaporated and 60 mL of CH₂Cl₂ wasadded to extract the product. The extract was then washed with water,dried with anhydrous Na₂SO₄, giving a green material after removal ofall solvent. Further purification was conducted using vacuum sublimation(160° C., 220 mtorr) and recrystallization from CH₂Cl₂ and hexane gave76 mg of Pt(bppz)₂ as bright green solid (0.13 mmol, 54%).

Spectra data of (5): MS (EI, 70 eV), observed m/z (actual) [assignment]:595 (595) [M⁺]. ¹H NMR (400 MHz, CDCl₃, 294 K): δ10.79 (d, ³J_(HH)=6.0Hz, 2H), 7.82 (ddd, ³J_(HH)=7.9 Hz, ³J_(HH)=7.6 Hz, ⁴J_(HH)=1.2 Hz, 2H),7.55 (d, ³J_(HH)=7.9 Hz, 2H), 7.19 (ddd, ³J_(HH)=7.6 Hz, ³J_(HH)=6.0 Hz,⁴J_(HH)=1.2 Hz, 2H), 6.49 (s, 2H), 1.42 (s, 18H). ¹³C NMR (125 MHz,CDCl₃, 294K): δ 161.4 (C_(py)), 155.4 (C_(pz)), 152.3 (CH_(py)), 149.4(C_(pz)), 138.8 (CH_(py)), 120.7 (CH_(py)), 117.8 (CH_(py)), 99.4(CH_(pz)), 32.6 (C_(t-butyl)), 31.0 (CH₃). Anal. Calcd. for C₂₄H₂₈N₆Pt:C, 48.40; N, 14.11; H, 4.74. Found: C, 48.31; N, 14.10; H, 4.88.

Example 6 Synthesis of Pt(fppz)₂ (6)

To the suspension of NaH (17 mg, 0.71 mmol) in 15 mL of THF was added3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH, 120 mg, 0.56 mmol)slowly at room temperature. After being stirred for 1 hour, the solutionwas filtered and transferred into a flask that loaded with 100 mg ofPt(DMSO)₂Cl₂ (0.24 mmol) and 10 mL of THF. The solution was refluxed for8 hours. Purification was conducted using sublimation (150° C., 200mtorr) and recrystallization from CHCl₃, giving 83 Mg of Pt(fppz)₂ asorange fine needles (0.13 mmol, 56%).

Spectra data of (6): MS (EI, 70 eV), observed m/z (actual) [assignment]:619 (619) [M⁺], 407 (407) [M⁺-fppz]. ¹H NMR (400 MHz, d-acetone, 294 K):δ 10.29 (d, ³J_(HH)=6.0 Hz, 2H), 8.19 (ddd, ³J_(HH)=7.6 Hz, ³J_(HH)=7.6Hz, ⁴ J_(HH)=1.4 Hz, 2H), 7.94 (d, ³J_(HH)=7.6 Hz, 2H), 7.47 (ddd,³J_(HH)=7.6 Hz, ³J_(HH)=6.0 Hz, ⁴J_(HH)=1.4 Hz, 2H), 7.067 (s, 2H). ¹⁹F(470 MHz, d-toluene, 294 K): δ−60.70l (s, CF₃). Anal. Calcd. forC₁₈H₁₀F₆N₆Pt: C, 34.90; N, 13.57; H, 1.63. Found: C, 34.44; N, 13.12; H,1.78.

Example 7 Synthesis of Pt(hppz)₂ (7)

To the suspension of NaH (13 mg, 0.54 mmol) in 15 mL of THF was added3-heptafluoropropyl-5-(2-pyridyl) pyrazole (hppzH, 150 mg, 0.48 mmol)slowly at room temperature. After being stirred for 1 hour, the solutionwas filtered and transferred into a reaction flask loaded with 100 mg ofPt(DMSO)₂Cl₂ (0.24 mmol) and 10 mL of THF. The solution was refluxed for12 hours and the solvent was completely removed under vacuum. The solidresidue was sublimed under vacuum (110° C., 200 mtorr). Furtherrecrystallization from CH₂Cl₂ at room temperature gave 113 mg ofPt(hppz)₂ as yellow needles (0.14 mmol, 58%).

Spectra data of (7): MS (EI, 70 eV), observed m/z (actual) [assignment]:820 (819) [M⁺]. ¹H NMR (500 MHz, d-THF, 294 K): δ 10.53 (d, ³J_(HH)=6.0Hz, 2H), 8.14 (ddd, ³J_(HH)=7.5 Hz, ³ J_(HH)=7.5 Hz, ⁴J_(HH)=1.0 Hz,2H), 7.95 (d, ³J_(HH)=7.5 Hz, 2H), 7.48 (ddd, ³J_(HH)=7.5 Hz,³J_(HH)=6.0 Hz, ⁴J_(HH)=1.0 Hz, 2H), 7.13 (s, 2H). ¹⁹F (470 MHz, d-THF,294 K): δ−80.83 (t, ³J_(FF)=8.5 Hz, CF₃), −109.06 (dd, ³J_(FF)=19.3 Hz,³J_(FF)=8.5 Hz, CF₂), −127.31 (s, CF₂). Anal. Calcd. for C₂₂H₁₀F₁₄N₆Pt:C, 32.25; N, 10.26; H, 1.23. Found: C, 32.06; N, 10.08; H, 1.41.

Example 8 Synthesis of Pt(bqpz)₂ (8)

A solution of potassium tetrachloroplatinate (K₂PtCl₄) (100 mg, 0.24mmol), 1-(5-tert-butyl-2H-pyrazol-3-yl)-isoquinoline (bqpzH, 130 mg,0.52 mmol) in a mixture of ethanol (15 mL) and water (5 mL) was heatedat 80° C. for about 16 h. After cooling the mixture to room temperaturethe precipitated solid was collected, washed with diethyl ether anddried under vacuum to give Pt(bqpz)₂ as orange solid (85 mg, 0.122 mmol)in 51% yield.

Spectra data of (8): MS (FAB), observed m/z (actual) [assignment] 696(696) [M⁺]. ¹H NMR (400 MHz, CD₂Cl₂, 294 K): δ 10.98 (d, J=5.6 Hz, 2H),8.82 (d, J=7.9 Hz, 2H), 7.95 (d, J=7.9 Hz, 2H), 7.86 (t, J=7.0 Hz, 2H),7.77 (t, J=7.4 Hz, 2H),7.66 (d, J=5.7 Hz, 2H), 7.06 (s, 2H), 1.55(s,18H). Anal. Calcd. for C₃₂H₃₂N₆Pt: C, 55.24; H, 4.64; N, 12.08. Found:C, 54.88; H, 4.94; N, 11.98.

Example 9 Synthesis of Pt(bzpz)₂ (9)

To the suspension of NaH (13 mg, 0.54 mmol) in 20 mL of THF was added3-tert-butyl-5-(2-pyrazine) pyrazole (bzpzH, 100 mg, 0.5 mmol) slowly atroom temperature. After being stirred for 1.5 hour, the solution wasfiltered and transferred into a reaction flask loaded with 100 mg ofPt(DMSO)₂Cl₂ (0.24 mmol) and 10 mL of THF. The solution was refluxed for12 hours and the solvent was removed under vacuum. The solid residue wassublimed under vacuum (180° C., 160 mtorr). Further recrystallizationfrom CH₂Cl₂ at room temperature gave 98 mg of Pt(bzpz)₂ as red fineneedles (0.16 mmol, 68%).

Spectra data of (9): MS (EI, 70 eV), observed m/z (actual) [assignment]:597 (597) [M⁺], 582 (582) [M-CH₃]. ¹H NMR (500 MHz, CDCl₃, 294 K): δ10.54 (d, ³J_(HH)=3.8 Hz, 2H), 8.83 (s, 2H), 8.38 (d, ³J_(HH)=3.8 Hz,2H), 6.50 (s, 2H), 1.43 (s, 18H). ¹³C NMR (125 MHz, CDCl₃, 294 K): δ162.4 (C_(py)), 149.7 (C_(pz)), 145.9 (C_(pz)), 144.2 (CH_(py)), 141.9(CH_(py)), 140.6 (CH_(py)), 100.9 (CH_(pz)), 32.7 (C_(t-butyl)), 30.9(CH₃). Anal. Calcd. for C₂₂H₂₆N₈Pt: C, 44.22; N, 18.75; H, 4.39. Found:C, 43.94; N, 19.21; H, 4.60.

Example 10 Synthesis of Pt(bmpz)₂ (10)

To the suspension of NaH (8 mg, 0.33 mmol) in 15 mL of THF was added3-tert-butyl-5-(5-methyl-2-pyrazine) pyrazole (bmpzH, 60 mg, 0.28 mmol)slowly at room temperature. After being stirred for 1.5 hour, thesolution was filtered and transferred into a reaction flask loaded with50 mg of Pt(DMSO)₂Cl₂ (0.12 mmol) and 10 mL of THF. The solution wasrefluxed for 10 hours and the solvent was removed under vacuum. Thesolid residue was sublimed under vacuum (180° C., 200 mtorr). Furtherrecrystallization from a mixture of CH₂Cl₂ and hexane gave 48 mg ofPt(bmpz)₂ as orange crystals (0.08 mmol, 65%).

Spectra data of (10): MS (EI, 70 eV), observed m/z (actual)[assignment]: 626 (625) [M⁺], 611 (610) [M⁺-CH₃]. ¹H NMR (500 MHz,CDCl₃, 294 K): δ 10.65 (s, 2H), 8.80 (s, 2H), 6.55 (s, 2H), 2.71 (s, 6H)1.43 (s, 18H). ¹³C NMR (125 MHz, CDCl₃, 294 K): δ 162.1 (C_(py)), 151.9(C_(pz)), 146.9 (C_(pz)), 146.5 (CH_(py)), 143.9 (CH_(py)), 139.5(CH_(py)), 99.9 (C_(pz)), 32.7 (C_(t-butyl)), 30.9 (CH₃), 21.6 (CH₃).Anal. Calcd. for C₂₄H₃₀N₈Pt: C, 46.07; N, 17.91; H, 4.83. Found: C,46.15; N, 17.70; H, 5.02.

Example 11 Synthesis of Pt(bptz)₂ (11)

To the suspension of NaH (16 mg, 0.67 mmol) in 20 mL of THF was added2-(5-tert-butyl-2H-[1,2,4]triazol-3-yl)-pyridine (bptzH, 110 mg, 0.5mmol) slowly at room temperature. After being stirred for 1.5 hour, thesolution was filtered and transferred into a reaction flask loaded with100 mg of Pt(DMSO)₂Cl₂ (0.24 mmol) and 10 mL of THF. The solution wasrefluxed for 10 hours and the solvent was removed. The solid residue wassublimed under vacuum (160° C., 300 mtorr). Further recrystallizationfrom a mixture of CH₂Cl₂ and pentane gave 90 mg of Pt(bptz)₂ as redorange crystals (0.15 mmol, 63%).

Spectra data of (11): MS (EI, 70 eV), observed m/z (actual)[assignment]: 597 (597) [M⁺]. ¹H NMR (400 MHz, CDCl₃, 294 K): δ 10.49(d, 3J_(HH)=6.0 Hz, 2H), 8.07(d, ³J_(HH)=7.6 Hz, 2H), 8.00 (ddd,³J_(HH)=7.6 Hz, ³J_(HH)=7.6 Hz, ⁴J_(HH)=1.2 Hz, 2H), 7.40 (ddd,³J_(HH)=7.6 Hz, ³J_(HH)=6.0 Hz, ⁴J_(HH)=1.2 Hz, 2H), 1.49 (s, 18H).Anal. Calcd. for C₂₂H₂₆N₈Pt: C, 44.22; N, 18.75; H, 4.39. Found: C,44.12; N, 18.69; H, 4.48.

Although the present invention has been described with reference tospecific details of certain embodiments thereof, it is not intended thatsuch details should be regarded as limitations upon the scope of theinvention except as and to the extent that they are included in theaccompanying claims. Many modifications and variations are possible inlight of the above disclosure.

1. A Pt(II) complex having the following formula:

wherein X₁ and X₂ independently are C or N; R₁ is H, C1–C8 alky, orC1–C4 perfluoroalkyl, R₂ is H, or R₁ and R₂ together are C4–C8 alkylene,or R₁ and R₂ together are bridged carbocyclic C4–C12 alkylene, when X₂is C; R¹ is H, C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ is omitted,when X₂ is N; R₇ is H or methyl, R₈ is H or methyl, or R₇ and R₈together are

when X₁ is C; R₇ is H or methyl, and R₈ is omitted, when X₁ is N.
 2. ThePt(II) complex as defined in claim 1, wherein X₁ is C, and R₇ and R₈together are


3. The Pt(II) complex as defined in claim 2, wherein X₂ is C.
 4. ThePt(II) complex as defined in claim 3, wherein R₁ and R₂ together arebridged carbocyclic C4–C12 alkylene.
 5. The Pt(II) complex as defined inclaim 4, wherein R₁ and R₂ together are

wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.
 6. The Pt(II)complex as defined in claim 5, wherein R₄, R₅ and R₆ are methyl.
 7. ThePt(II) complex as defined in claim 3, wherein R₁ is C1–C8 alkyl, orC1–C4 perfluoroalkyl, and R₂ is hydrogen.
 8. The Pt(II) complex asdefined in claim 2, wherein X₂ is N, R₁ is C1–C8 alkyl, or C1–C4perfluoroalkyl, and R₂ is omitted.
 9. The Pt(II) complex as defined inclaim 1, wherein X₁ is C, R₇ is H or methyl, and R₈ is H.
 10. The Pt(II)complex as defined in claim 9, wherein X₂ is C.
 11. The Pt(II) complexas defined in claim 10, wherein R₁ and R₂ together are bridgedcarbocyclic C4–C12 alkylene.
 12. The Pt(II) complex as defined in claim11, wherein R₁ and R₂ together are

are wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.
 13. The Pt(II)complex as defined in claim 12, wherein R₄, R₅ and R₆ are methyl. 14.The Pt(II) complex as defined in claim 10, wherein R₁ is C1–C8 alkyl, orC1–C4 perfluoroalkyl, and R₂ is hydrogen.
 15. The Pt(II) complex asdefined in claim 10, wherein R₁ and R₂ together are C4–C8 alkylene. 16.The Pt(II) complex as defined in claim 10, wherein R₁ and R₂ togetherare tetramethylene.
 17. The Pt(II) complex as defined in claim 9,wherein X₂ is N, R₁ is C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ isomitted.
 18. The Pt(II) complex as defined in claim 1, wherein X₁ is N,R₇ is H or methyl, and R₈ is omitted.
 19. The Pt(II) complex as definedin claim 18, wherein X₂ is C.
 20. The Pt(II) complex as defined in claim19, wherein R₁ and R₂ together are bridged carbocyclic C4–C12 alkylene.21. The Pt(II) complex as defined in claim 20, wherein R₁ and R₂together are

are wherein R₄, R₅ and R₆ independently are C1–C4 alkyl.
 22. The Pt(II)complex as defined in claim 21, wherein R₄, R₅ and R₆ are methyl. 23.The Pt(II) complex as defined in claim 19, wherein R₁ is C1–C8 alkyl, orC1–C4 perfluoroalkyl, and R₂ is hydrogen.
 24. The Pt(II) complex asdefined in claim 19, wherein R₁ and R₂ together are C4–C8 alkylene. 25.The Pt(II) complex as defined in claim 24, wherein R₁ and R₂ togetherare tetramethylene.
 26. The Pt(II) complex as defined in claim 18,wherein X₂ is N, R₁ is C1–C8 alkyl, or C1–C4 perfluoroalkyl, and R₂ isomitted.